The invention relates to solid state electronic induction heating for cooking, and more particularly to transistor apparatus for such purpose.
It is known from U.S. Pat. No. 3,806,688, issued Apr. 23, 1974 in the name of the same assignee as the assignee of this application, to generate eddy currents at ultrasonic frequency in a metallic utensil for cookware. The heating coil used is part of a resonant inductance-capacitance circuit maintained at resonance by a transistor oscillator driven by feedback from the resonant circuit. The patent also teaches control of the power fed from the chopper to the heating coil by a control signal representing the excursions of the resonant voltage at the coil beyond the direct current voltage applied to the chopper. The DC voltage is automatically adjusted to meet the pan temperature as required by the user.
Induction cooking offers many advantages over conventional cooking, such as the electric range. The most typical advantages are safety for the user and a more efficient transfer of energy from the heating spiral to the cooking utensil. However, induction cooking requires sophisticated electronic equipment and such added sophistication must be matched in terms of cost and reliability with the more simple technique of conventional ranges. Therefore, the merit of induction cooking from an industrial and commercial point of view resides essentially in the basic design of the circuitry, the ruggedness of the construction, the relative simplicity of the solid state arrangement and the choice of the constructive elements.
The technique applied according to the description in the U.S. Pat. No. 3,806,688 is attractive from this point of view since transistors are used instead of thyristors. Transistors can be turned off by the control electrode, whereas thyristors must be forced off by bringing the anode current to zero. Besides a gating circuit is not required for timing the conduction periods of a transistor. As described in the patent, the resonant heating coil itself is used by feedback to alternately switch one transistor at a time when the collector current of the other passes by zero. Also, power control is provided by a feedback loop around the transistors for adjusting the DC voltage applied to the transistor oscillator in accordance with the excursions of the resonant voltage beyond the applied DC voltage. The latter constitutes an excellent indicator of the relation existing on the heating coil between the voltage supplied from the DC line and the energy drawn by eddy currents in the cooking utensil. However, cost reduction makes it desirable to provide a more compact circuitry for the control loop.
The object of the present invention is to provide an improved induction cooking apparatus.
Another object of the present invention is to provide improved power control for solid state induction cooking apparatus normally operating at or near resonance.
A further object of the present invention is to provide a frequency controlled induction cooking apparatus.